The present invention relates to room temperature vulcanizable silicone rubber compositions and more particularly relates to a neutron absorbing room temperature vulcanizable silicone rubber composition either of the one-component or two-component type.
Nuclear reactors for various types of uses are well known such as, for instance, nuclear reactors for the providing of electric power, etc. As is well known in the art, such nuclear reactors have to have the proper type of shielding to protect workers in the plants from the harmful rays produced in the nuclear reactions, as well as to prevent undue contamination of other physical parts of the plant other than the nuclear reactor itself. Accordingly, in the fabrication of such nuclear reactors it is well known to have lead shields to protect workers in the plant from harmful gamma rays. In such nuclear reactions there is also produced what is known as fast neutrons. Such fast neutrons which are emitted in the nuclear reaction usually pass conventional barriers and are a safety threat to workers in the plant. Also, such fast neutrons have the potential of causing normally non-radioactive metals to become radioactive.
It has been known in the art to utilize in such nuclear plants certain moderators; a moderator being a material of a high hydrogen density which slows down the fast neutrons but does not absorb them. One example of a good moderator is, for instance, water. However, as stated previously, water does not absorb the fast neutrons but just slows them down. Accordingly, it would be highly desirable to have in such nuclear plants shielding in which the shielding performs the function of both a moderator for slowing down the fast neutrons and also as an absorbent of most of the neutrons without giving off large amounts of radioactive energy or thermal energy.
Accordingly, fabricators of such nuclear plants are constantly looking for effective shielding which would perform the function of both being a moderator and an absorber of fast neutrons.
Room temperature vulcanizable silicone rubber compositions are well known. These compositions generally comprise three distinct types of room temperature vulcanizable silicone rubber compositions. First, there is the one-component system which generally comprises as the basic ingredients, a silanol end-stopped diorganopolysiloxane polymer and acetoxy or alkoxy functional silanes, filler, a catalyst which in the case of the acetoxy functional system is the metal salt of the carboxylic acid and which in the case of the alkoxy functional system is preferably a titanium catalyst. Other well known additives may be added to such compositions, for instance, to make them self-bonding or to give them other properties. The ingredients of such one-component room temperature vulcanizable silicone rubber compositions are usually mixed in the anhydrous state and packaged as such so that they are protected from contacting atmospheric moisture. Such compositions when applied and allowed to come in contact with atmospheric moisture cure to a silicone elastomer with generally the properties of having good weatherability characteristics, water repellent properties, as well as high and low temperature stability.
Another type of room temperature vulcanizable silicone rubber composition is a two-component type which comprises one-component comprised of a silanol end-stopped diorganopolysiloxane polymer, etc., and other necessary additives, and the second component usually comprising an alkyl silicate or a partial hydrolysis product of an alkyl silicate with a metal salt of carboxylic acid as a catalyst. Other ingredients may be added to these two components as is desired. The composition is packaged in two components and stored until it is desired to cure the mixture. When it is desired to cure the two components into a silicone elastomer, the two parts are simply mixed and fabricated into the part desired and allowed to cure to a silicone elastomer with properties comparable to the silicone elastomer produced by the one-component system. Such two-component system will cure either in the presence or absence of atmospheric moisture. Such compositions, that is, the one-component system that produces silicone elastomer or the two-component system that produces silicone elastomer are utilized depending on the type of application.
Another well known two-component room temperature vulcanizable silicone rubber composition comprises vinyl containing polysiloxane into which there is mixed a filler and other ingredients and which is stored as a separate component and a second component comprising a hydride silane or siloxane in combination with a platinum catalyst, which may be either solid platinum deposited on a solid carrier or a platinum complex which is more preferred. Such composition cures when the two components are mixed to a silicone elastomer with the desired properties, which composition also has flame inhibitiveness because of the platinum that is present in the composition. Such vinyl containing polysiloxanes, two-component systems, can be cured either in the presence or absence of moisture either at room temperature for a prolonged period of time, such as 24 hours, or can be cured in a matter of minutes at elevated temperatures, such as temperatures above 100.degree. C.
A particular type of room temperature vulcanizable silicone system that is used for a particular shielding or coating will depend, of course, on whether it is desired to have the mixing of the ingredients at the construction site, as is necessary with the two-component system or if it is desired that the components be already mixed such as the one component system so that the material can be simply applied to form whatever part is necessary.
Also, the physical properties of such systems will vary from one to the other depending on the other types of ingredients in the composition and depending, of course, on the basic properties of the system itself as distinguished from each other. It is well known that such room temperature vulcanizable silicone rubber compositions could be used because of their high hydrogen density to act as moderators for fast neutrons, that is, these compositions have the ability to slow down fast neutrons when utilized as a shielding. However, such traditional room temperature vulcanizable silicone rubber compositions did not have any neutron absorbing properties. Accordingly, they could not be utilized to form a shielding for nuclear reactors when it was desired that such shielding would have both moderating and absorbing properties.
Accordingly, it is one object of the present invention to provide for a one-component room temperature vulcanizable silicone rubber composition which is both a neutron moderator and a neutron absorber.
It is an additional object of the present invention to provide for a two-component room temperature vulcanizable silicone rubber composition which is both a neutron absorber and a neutron moderator.
It is yet an additional object of the present invention to provide for a two-component room temperature vulcanizable silicone rubber composition which utilizes an SiH-olefin platinum catalyzed reaction which two-component system performs both as a neutron moderator and absorber by incorporation into the composition as a boron compound or a boron powder.
These and other objects of the present invention are accomplished by means of the disclosure set forth herein below.